I try to spread the word on Stack Overflow awesomeness whenever I get a chance. How about providing a "Recruiter badge" which you obtain when you've managed to get XX (10, 50 or 100) number of people to sign up and they reach a certain level of reputation like 50 or 100... something to indicate they’re active on the site?

This is basically a simple referral scheme: when you sign up, you can enter the ID of the SO user who was kind enough to tell you about the site.

UPDATE

To all the downvoters: don't we want to reward behaviour that benefits the community?

Do you think you are more likely to get new people with better quality questions & answers? Or are you going to get more people who will create fake accounts to game the system?
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Christian PayneMay 17 '10 at 3:53

1

I think it's pretty easy to prevent gaming. And I also think the more people who knows about SO the more likely it is the site will attract people with quality questions and answers.
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Jakob GadeMay 17 '10 at 15:05

Presumably, it would be more like a signup trial system - if the user stayed on for X number of months and actually contributed to SO (earned X amount of rep in the time alloted, or whatever), then the referrer would gain the point towards the badge.
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wompJul 6 '09 at 5:43

3

Maintaining 50+ zombie accounts for achieving this badge would be a lot of work, and I suppose it would be pretty easy to detect as well. I think it would be pretty easy to prevent gaming on this one. Maybe a combination of time AND rep would do the trick, just like womp suggested...
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Jakob GadeJul 6 '09 at 6:29

1

The more I've considered this badge the more I like the idea given an implementation that prevents gaming.
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ahsteeleJul 6 '09 at 16:43

For meta you'd just have to post 100 questions about unicorns. 50 of those would be considered funny and upvoted.
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EarlzMay 17 '10 at 2:14

I always hate these sorts of rewards, because I tend to recommend websites to people via the front page - I've found they're much less likely to click if I point them to a link that's obviously benefiting me in the process.

(Also that most of the people that I've recommended it to have already signed up by now...)

There are lots of ways you could make it easy and painless to point out who should have the referal "point", and it doesn't have to be done during the initial registration. That way all your friends can still help you towards this badge. :)
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Jakob GadeJul 6 '09 at 15:36

How about requiring people transfer rep for introducing trusted colleagues.

Sort of like Gmail invites, but you have to spend rep. points to recommend them. If I know a good C/Java/database developer, I click 'something' that sends them a 'try out StackOverflow' email and gives them an initial 200 rep points (enough for them to reach the 'reduced advertising' level).

The 200 rep come off my balance, which reduces the temptation to 'game' the system, but I gain the recruiter badge (maybe a 'counted' badge so I can have five if I've recommended fifty people).

The introduced member gets a benefit too, so they may be more likely to sign up.